Having trouble with 04 dodge stratus SE overheating

My stratus overheated today and cut off on me twice. It wil still start and run for a
few minutes before overheating again. I have minimal car knowledge but i have
checked to make sure that their us not a coolant leak into our engine the oil
consistency is viscous as it should be no water in the oil, however, it will not
maintain the antifreeze level. I havent seen any leaking under the car, nor into the
floorboards. The car is still being paid for and i have been advised to contact the
dealership and request one of their mechanics come out and take a look at it. We
have only had the care for a little over a year. My dad suggested that it could be
the thermostat, or blown headgasget, or the waterpump how can i find out what the
cause is? What kind of prices am i looking at for repair for each of these issues? As
far as i know a headgasget repair would be the most expensive, at around $1,000 or
more, and waterpump at about $500+ can anyone confirm or ballpark these values
for me?